Electron Beam Writing (EBW) technology plays an important role in IC fabrication. It is used as a main tool for lithography mask writing. Recently its variation, the Electron Beam Direct Writing (EBDW) technology, has been considered as a prominent candidate for low volume IC production at 90 nm and below. These approaches have become solutions that are used to address mask cost issues for low volume LSI production.
The major weakness of this technology is its relatively low throughput. To overcome this barrier, the Variable Shape Beam (VSB) and the Cell Projection (CP, alternatively called Character Projection, or Block Projection) techniques have been introduced. The cell projection technique uses a stencil that allows writing of complicated repetitive patterns (CP cells) by one exposure shot, thus decreasing overall exposure time and increasing the writing system throughput. To utilize capabilities of the CP technique, a stencil design system has to be developed that is capable of proper repetitive pattern extraction, CP cell design and optimum CP cell placement on the stencil.
As shown in FIG. 1, this technique allows writing complicated patterns by one shot with an electron gun 100 that produces an electron beam 102, thus increasing throughput of the system. As noted, a central part of the technique is a stencil 110 that consists of a number of stencil patterns 112, alternatively called characters or CP cells. To increase the throughput and accuracy of a CP system, proper stencil pattern design is necessary. This technique produces an image 120 of a selected stencil pattern 114 on a substrate 122.
Although EBW technology provides many advantages, it is not perfect. Various effects negatively affect the process quality, decreasing yield and lowering overall throughput. Certain effects like Coulomb blur, proximity effect, electron beam (EB) distortions can be minimized by proper stencil design. Such processing may include, for example, more detailed fracturing and dose calculation, proper CP cell design and stencil layout. However, such calculations lead to the increase of the data volume and writing time. Also, making such calculations for the whole writing pattern is a time and resources consuming task.
As an enhancement of the CP writing technique, there exists the partial exposure method. In this method, a part of a CP cell can be illuminated, allowing writing not only whole CP cell but also its part by one shot. The advantage of this method is that one CP cell can be used for writing different patterns. Therefore, the same stencil can be used for writing more patterns by one shot.
The conventional partial CP exposure method has disadvantages. To expose a part of a CP cell, there must be a blank region around the cell that blocks the rest of the beam, as in most EB writing systems, the size of the beam illuminating the stencil is fixed by the first aperture. If another CP cell appears in the region that is supposed to be blank, a part of it will be exposed. Therefore, in the conventional partial CP exposure art, the cells are less densely packed than those could be without using partial CP exposure, because partial CP cells must have enough surrounding blank space.
Automation of the data preparation process for EBDW systems is a problem that has not been satisfactory solved yet. To achieve good throughput, repetitive design patterns must be selected and processed, then placed as CP cells on the stencil. Simultaneously, quality issues must be properly taken into account to maintain acceptable yield.
In current systems, the above mentioned problems are partially solved by introducing specific design flows and data processing algorithms for each specific design. The development of a layout specific stencil design system significantly increases the overall development time, therefore making the technology less advantageous.
There are also problems related to feature miniaturization in semiconductor devices. With the feature size decrease, writing accuracy becomes more and more important. But required accuracy is not uniform for all parts of a design. Some parts require tighter tolerances, some parts require less accuracy. The required accuracy depends on the feature's design intent. Current EBDW systems do not have capabilities for design intent aware operation.
Geometric structure of a CP cell and its position on the stencil strongly affect writing quality. To achieve required quality and simultaneously maximize the usage of stencil area, careful CP cell extraction and stencil layout optimization are needed that take the design intent information into account. Current data preparation systems do not support this functionality.